Apparatuses are known for heat treatment of various monofilaments, narrow tapes or belts, packing tapes, multifilaments, tapes and other webs of plastics. Such apparatuses comprise a cover member and a bottom member hingedly interconnected and defining a hot-air channel when closed and optionally interlocked, said hot-air channel extending from an inlet at one end of the apparatus to an outlet at the opposite end of said apparatus. The apparatus comprises equipment for providing a flow of hot air at a suitable temperature which can be up to 300° C., but which is usually in the range of from approximately 180 to 200° C. While passing through the hot-air channel, the article in question is subjected to a suitable stretching, which in combination with the heat ensures the desired processing. The apparatuses of this type can be up to 6 m long and present an internal width inside the hot-air channel of up to approximately 2 m. Such an apparatus is usually used for a continuous processing of the articles in question and can operate non-stop for a very long period of time, such as approximately 1 month. A good result depends on a uniform action, but the action can be disadvantageously affected in case it is necessary for some reason to open the cover member, such as in case an unsatisfactory number of articles advanced in parallel through the hot-air channel are damaged and it is desired to introduce fresh articles in order to ensure the productive capacity. As a result, all the articles are damaged and must therefore be replaced.